Optical light guides made of a transparent material having substantially planar inner surfaces and outer surfaces which are "in octature" have been utilized to transport light, as illustrated in U.S. Pat. No. 4,260,220 to Whitehead. These devices are typically constructed of an optical lighting film made of flexible polymeric sheets of a transparent material having a structured surface on one side and a smooth surface opposite the structured surface. The structured surface of the devices preferably include a linear array of miniature substantially right angled isosceles prisms arranged side-by-side to form a plurality of peaks and grooves. Further, the perpendicular sides of the prisms make an angle of approximately 45 degrees with the smooth surface. This structure of the polymeric sheets, as well as the shape of the light conduit, enables light to be constrained to travel through the light conduit without escaping through its walls if the angle by which the light rays deviate from the longitudinal axis of the light conduit does not exceed a maximum angle, or critical angle, which depends upon the refractive index of the light conduit material. Thus, light entering a light conduit at an angle less than this predetermined maximum is totally internally reflected. These light conduits have been constructed in various cross-sections, such as square cross-sections, as illustrated in U.S. Pat. No. 4,260,220, and circular cross-sections, as illustrated in U.S. Pat. No. 4,805,984.
In many applications, it is desirable to make light escape from the light conduit in a controllable manner. Many means for facilitating emission of light from the light conduit have been used in the past. For example, an extractor such as a diffuse scatterer made from a highly reflective white polymeric tape such as SCOTCHCAL ELECTROCUT brand film, manufactured by 3M Company, St. Paul, Minn., may be placed inside a light conduit to increase the rate of leakage, or emission, of the light from inside the light conduit. The diffuse scatterer increases the rate of leakage by "scattering" light that hits it into non-total internal reflecting angular regions of the light conduit, thereby increasing the amount of light in those angles which allow light to be emitted from the light conduit. Typically, a strip of the highly reflective white polymeric tape is placed over the length of the light conduit to cause the scattering. Thus, an additional component, namely the diffuse scatterer, must be added to the light conduit to increase the rate of leakage.
While a diffuse scatterer increases the rate of leakage from inside a light conduit, only light which actually hits the diffuse scatterer will be scattered and thereby increase the rate of leakage. In light conduits with long aspect ratios, that is, for a tubular light conduit with a length much greater that its diameter, the diffuse scatterer may run the entire length of the light conduit. The rate at which the light is emitted from the light conduit is higher in the angular regions of the light conduit where an extractor, such as a diffuse scatterer, is placed than in angular regions of the light conduit where the light is never reflected from the diffuse scatterer. The effectiveness of the diffuse scatterer is reduced as the length of the light conduit increases because after light is emitted out of the light conduit, depleted zones are formed in angular regions associated with the diffuse scatterer and therefore the diffuse scatterer reflects less light. Nonetheless, an extractor allows a greater amount of light to be emitted from the light conduit than a light conduit without an extractor.
Another means for increasing the amount of light emitted from the light conduit is including a non-planar surface, a rough outer surface or round corners in an outer corrugated surface to increase the amount of light emitted from those surfaces, as illustrated in U.S. Pat. No. 4,615,579 to Whitehead.